A fifth lane will open Thursday on a six-mile stretch of the eastbound Riverside Freeway. It could help ease congestion for Orange County-Inland Empire commuters.

Motorists braving the heavily traveled 91 corridor between the Inland Empire and Orange County could see their commute ease as of Thursday.

Orange County officials Thursday will inaugurate a new eastbound lane on a six-mile section of the Riverside Freeway, which commuters will be able to use starting that morning, officials said.

The yearlong, $65-million widening project was the first in Orange County funded largely by the massive federal stimulus package enacted in 2009.

"Any relief we can provide for those commuters is going to be very welcome," said Joel Zlotnik, spokesman for the Orange County Transportation Authority.

The new lane will increase the eastbound lanes from four to five between the 241 toll road and the 71 Corona Expressway. There are already toll lanes down the center of the freeway in both directions.

Transportation officials say the extra lane should make it easier for the estimated 300,000 commuters who creep along the route each day. Many live in Inland Empire bedroom communities but hold jobs in Orange or Los Angeles counties. In addition, the freeway is a major gateway for motorists heading to area ski resorts or Las Vegas.

Dijana Vandenheuvel, 32, is unconvinced the addition of a single eastbound lane will do much to relieve congestion on a highway she considers "an absolute nightmare."

"I always try to avoid the 91, no matter what," said Vandenheuvel, a Huntington Beach resident who uses the 91 to deliver parts for an aircraft manufacturer. "The new lane may help a little bit, but I personally don't think it will solve the problem because it's so congested."

The 91 bottleneck has vexed planners for decades. A partial answer was provided a decade ago with the opening of toll lanes along a portion of the freeway in Orange County, offering commuters a quicker pay-as-you-go method. Last year, new lanes were added to the 241 tollway, which cuts from the 91 to south Orange County.

There isn't a lot of room to add more toll-free lanes because the 91 runs through a narrow canyon amid several mountain ranges. In recent years, planners have talked about tunneling 11.5 miles through the Cleveland National Forest to build a new route into Orange County.

But those plans on are hold because there is no money for the estimated $8.6-billion project, Zlotnik said.

County transportation leaders will gather at 10 a.m. Thursday at 8712 E. Santa Ana Canyon Road, just off the 91, to dedicate the new lane.